Afterburner seals with heat rejection coats

ABSTRACT

A method for applying a coating system that is applied to a surface of a component for preventing or at least substantially preventing interdiffusion between the component surface and a protective thermal layer applied to the component surface when the thermal layer is exposed to elevated temperatures. The method includes applying a carrier layer containing aluminum to the component surface. Next, the layer is heated to a first predetermined temperature for a first predetermined period of time in the substantial absence of oxygen to bond the aluminum with the component surface, the heat dissolving the carrier portion of the aluminum layer. The remaining portion of the aluminum layer is then heated to a second predetermined temperature for a second predetermined period of time to form an oxidized aluminum layer. Finally, at least one protective thermal layer is applied over the oxidized aluminum layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application is related to Application No. ______, AttorneyDocket No. 13DV-14208, filed contemporaneously with this Application onDec. 20, 2002, entitled “COMBUSTION LINER WITH HEAT REJECTION COATS”assigned to the assignee of the present invention and which isincorporated herein by reference, and to Application No. ______,Attorney Docket No. 13DV-14209, filed contemporaneously with thisApplication on Dec. 20, 2002, entitled “TURBINE NOZZLE WITH HEATREJECTION COATS” assigned to the assignee of the present invention andwhich is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] Thus there is a need in the art for an inexpensive, lightweightmeans to prevent interdiffusion between component surfaces and heatrejection coatings. The present invention relates generally to heatrejection coats applied to component surfaces exposed to hightemperatures and more particularly to providing a diffusion barriersub-coating prior to applying the heat rejection coats to the componentsurfaces to stabilize and preserve the heat rejection coats.

BACKGROUND OF THE INVENTION

[0003] Components exposed to elevated temperatures and mechanicalstresses, such as aircraft engines which typically employ nickel, ironor cobalt based superalloys, require protective coatings from corrosionand from the high operating temperatures to achieve reliable operationfor extended periods of time. More specifically, component surfaceshaving metallic heat rejection coatings, such as platinum, gold orrhodium which may be sandwiched between a pair of stabilizing layers,such as tantalum, that are exposed to radiative flames exhibit bothmeasurable temperature decrease and increased service life compared touncoated component surfaces. These heat rejection coatings achieve thistemperature decrease by effectively reflecting the radiative energy awayfrom the component surface. Accordingly, it is highly desirable to applythese heat rejection coatings to similarly exposed surfaces. However,this is not possible for certain metal alloy parts, such as afterburnerseals, which may be regularly exposed to temperatures exceeding about788° C. (1450° F.). At this temperature range, the heat rejectioncoating interdiffuses with the underlying component surface, orsubstrate. In essence, a portion of the heat rejection coating materialmigrates into the component substrate material. This interdiffusioncauses the reflective heat rejection surface to become a radiationabsorber, losing or at least substantially losing its ability to reflectradiative energy, resulting in a reduction of its ability to decreasecomponent surface temperature, thereby decreasing the service life ofthe component. Therefore, a means to prevent the interdiffusion betweencomponent surfaces and the heat rejection coatings is highly desired.

[0004] One method to prevent this interdiffusion is the provision of abarrier coating applied between the component surface and the heatrejection surface. A variety of these barrier coatings are known in theart and include paint-on dielectric oxides, chemical vapor depositedoxides and baked-on rare earth oxides. However, none of these barriercoating constructions may be utilized in this application because theyeither are inefficient in preventing diffusion or lose their adhesiveproperties at higher temperatures.

[0005] Alternately, it has been shown that nozzles may be covered with athick macroscopic coating of a ceramic thermal barrier coating, referredas TBC, which is also known as “smooth coat,” commonly employing a TBCcomposition referred as “(AJ11).” U.S. Pat. Nos. 5,624,721 and 5,824,423are directed to methods which employ aluminum bond coat layers forsecuring TBC coatings. While heat rejection coatings have been shown toremain intact when applied over these TBC coatings, the thick TBCcoatings are expensive to manufacture and apply and are extremely heavy,effectively limiting their application to aerospace components.

[0006] Thus there is a need in the art for an inexpensive, lightweightmeans to prevent interdiffusion between component surfaces and heatrejection coatings.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a method for applying acoating system that is applied to a surface of a component forpreventing or at least substantially preventing interdiffusion betweenthe component surface and a protective thermal layer applied to thecomponent surface when the surface is exposed to elevated temperatures.The method includes first applying an aluminum-based material to thecomponent surface. In addition to aluminum, this material may include acarrier material and a binder, both of which are typically organic whenthe material is a paint. Next, the layer is heated to a firstpredetermined temperature for a first predetermined period of time inthe substantial absence of oxygen to metallurgically bond the aluminumwith the component surface, the heat volatizing the carrier and binderportion of the aluminum layer. The remaining portion of the aluminumlayer is then heated to a second predetermined temperature for a secondpredetermined period of time in the presence of oxygen to form anoxidized aluminum layer alumina. Finally, at least one protectivethermal layer is applied over the alumina.

[0008] The aluminum layer can be applied by standard commerciallyavailable aluminide processes whereby aluminum is reacted at thesubstrate surface of the component to form an aluminum oraluminum-containing composition which provides a reservoir for thegrowth of the aluminum oxidation layer. This aluminum layer is typicallyand predominantly aluminum, but may also be combined with other metals,including nickel, cobalt and iron as well as aluminum phases of nickel,cobalt and iron, or may be formed by contacting an aluminum vaporspecies or aluminum rich alloy powder with the component substrate anddepositing the aluminum on the substrate surface. This layer istypically metallurgically bonded to the substrate and may beaccomplished by numerous techniques, including a pack cementationprocess, over-the pack processing, spraying, chemical vapor deposition,electrophoresis, sputtering, vapor phase aluminiding and slurrysintering with an aluminum rich vapor and appropriate diffusion heattreatments. Aluminum will form highly stable refractory oxide layers atthe operating temperature of hot section components which are tightlyadherent and cohesive and thus effective to block incursions ofcorrosive chemical agents into the component substrate, so long as thealuminum oxide layer remains intact while preventing the migration ofsubstrate elements outward. When a coating is applied over the alumina,it prevents the coating elements from migrating inward or the substrateelements from migrating outward. In other words, the oxide layer willact as a barrier to prevent interdiffusion of elements across it.

[0009] A primary advantage of the present invention is an inexpensive,lightweight means to prevent interdiffusion between the substrate andheat rejection coatings applied over the substrate surfaces exposed toelevated temperatures.

[0010] Other features and advantages of the present invention will beapparent from the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic cross sectional view of a jet enginedepicting component regions having surfaces suitable for employment ofthe method for applying a coating system of the present invention.

[0012]FIG. 2 is an enlarged partial cross sectional view taken from FIG.1 of the afterburner region after a paint coating containing aluminumhas been applied.

[0013]FIG. 3 is the enlarged partial cross sectional view of FIG. 2 ofaluminum diffusing into the substrate by application of heat in thesubstantial absence of oxygen.

[0014]FIG. 4 is the enlarged partial cross sectional view of FIG. 3 offormation of an aluminum oxide layer forming from migration of thediffused aluminum from within the substrate by the application of heatin the presence of oxygen.

[0015]FIG. 5 is the enlarged partial cross sectional view of FIG. 4after application of a heat rejection coating over the aluminum oxidelayer.

[0016] Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to FIG. 1, a jet engine 10 is provided having a hotsection 14 which advantageously utilizes the coating system and methodof application of the present invention. Engine 10 includes in serialflow relation proceeding in a direction from an inlet 16 to exhaustnozzle 30, through a low pressure compressor 18, a high pressurecompressor 20, a combustor 22, a high pressure turbine 24, a lowpressure turbine 26, an afterburner 28, and terminating in exhaustnozzle 30. Afterburners 28 are optional items used in militaryapplications. Air entering inlet 16 is compressed by compressors 18, 20before reaching combustor 22 where the highly compressed air is mixedwith fuel and ignited. This air/fuel exhaust mixture is then propelledthrough turbines 24, 26 which are urged into rotation by the passingmixture of hot gases to likewise rotatably drive respective compressors18, 20, since these components are connected to a common drive shaft.Upon reaching optional afterburner 28, fuel is introduced into themixture stream to augment thrust, utilizing the unburned oxygen in theexhaust gas to support combustion. Afterburners are typically used inmilitary aircraft. They increase the speed of the aircraft for shortperiods of time, by injecting fuel into the hot exhaust gas stream whereit is combusted, thereby providing additional thrust. The temperature ofthe afterburner flame can exceed about 1,700° C. (about 3,100° F.), sothe burners of afterburner 28 are directed radially inward so that atleast a portion of the mixture from turbines 24, 26 flow past the wallof this region, helping maintain wall temperatures at somewhat reducedtemperature levels. The resultant increased temperature of the exhaustgas increases its velocity as it leaves exhaust nozzle 30, providingincreased engine thrust.

[0018] Afterburner 28 components, such as the seals, are nonethelesssubjected to significant radiative heat from afterburner flames despitethe burner orientations and will greatly benefit from the presentinvention.

[0019] Referring to FIG. 2, a sub coating layer 52 which is a carrierlayer containing aluminum applied over the substrate 50 shall now bediscussed. In the preferred embodiment, sub coating layer 52 is formedfrom application of commercially available spray paint, such as Krylon®No. 1404, manufactured by Sherwin-Williams Company, although comparablepaints from other manufacturers could likely also be used. It isrealized that any of the conventional methods to form this aluminumlayer previously discussed could also be employed. Carrier materialcontained within the layer permits sub coating layer 52 to be sprayedover a substrate surface 51 of a substrate 50, such as an afterburnerseal. Sub coating layer 52 may be applied to substrate surface 51 ofsubstrate 50 in a manner substantially similar to that employed to applya coat of paint to an article sufficient to “cover” the article. Inother words, by applying one, preferably two, coats of paint from acommercially available spray can to surface 51, sub coating layer 52contains an amount of aluminum particles 54 sufficient to ultimatelyform an aluminum oxide layer 56 (FIGS. 4, 5) on surface 51 as will bediscussed in greater detail below.

[0020] Further referring to FIG. 2, which is a partial cross-sectionalview of a coated afterburner 28 seal, aluminum particles 54 carriedwithin sub coating layer 52 are suspended within binder materials (notshown) in the paint formulation which bind the layer to the seal surfaceand prevent aluminum particles 54 from combining with oxygen to formaluminum oxide. These binder materials will be removed, i.e., vaporized,by a first thermal treatment step that will be further discussed belowand is not otherwise addressed herein. Aluminum particles 54 preferablyhave a platelike morphology that will be substantially oriented parallelto surface 51. More preferably, aluminum particles 54 are about ½ micronin thickness and are substantially equally distributed within subcoating layer 52. These particles preferably have an aspect ratio ofbetween about 100:1 to about 10:1, 20:1 being the most preferred.Particle aspect ratios exceeding this upper range are difficult to applyby spraying, and ratios below this lower range have decreased“coverability.”

[0021] In preparation for this first heating step, substrate 50 isplaced in an environment, such as a substantially fluid-tight oven orheating chamber, having an extremely low oxygen partial pressure, orhaving a substantial absence of oxygen. This may also be accomplished byan environment filled with inert gases, such as argon, helium, or evenhydrogen or nitrogen, although a vacuum of sufficient magnitude may beemployed. Once substrate 50 has been placed in the desired environmentor the conditions of the desired environment have been achieved,substrate 50 and sub coating layer 52 are subjected to a first heattreatment. During the first heat treatment, the environment temperaturereached and maintained for the duration of the first heat treatment mayrange from about 600° C. (1,100° F.) to about 1,000° C. (1,830° F.). Onehaving skill in the art realizes that the duration of the first heattreatment varies depending upon the temperature selected, since the rateof diffusion of aluminum is exponentially affected by temperature, forexample, substrate 50 will typically require about fifty hours ofexposure at about 600° C. (1,100° F.), or about one hour of exposure atabout 1,000° C. (1,830° F.) to achieve substantially the same results,i.e., same depth of diffusion. Therefore, any number of heat/exposurecombinations may be employed as a matter of manufacturing convenience,so long as the results achieved substantially mirror the results of the600° C./1,000° C. (1,110-1,830° F.) exposures just described. Once thisfirst heat treatment has been completed, referring to FIG. 3, asignificant amount of diffused aluminum 55 is diffused into substrate50, forming an alloy with the interdiffused substrate.

[0022] Referring to FIG. 4, after the first heat treatment has beencompleted, substrate 50 is subjected to a second heat treatment. Inessence, termperature/exposure of the second heat treatment issubstantially similar to that previously described for the first heattreatment. However, the major difference between the two heat treatmentsis that the second heat treatment is performed in the presence ofoxygen. This oxygen exposure promotes the formation of an aluminum oxidelayer 56 along the surface of substrate 50. Aluminum 54 remaining on thesurface oxidizes and a portion of the diffused aluminum 55 that hadpreviously diffused into substrate 50 during the first heat treatmentmigrate to the substrate surface so as to form a continuous tightlyadherent aluminum oxide layer. Preferably, aluminum oxide layer 56 isfrom about one to about ten microns thick, although this layer maypermissibly be up to about ten mils (0.010 inches) in thickness.

[0023] Referring to FIG. 5, after aluminum oxide layer 56 has beenformed, a smooth protective thermal coating may be applied. This coatingmay be chemical vapor deposited via a reagent of tantalum ethoxide whichflows into the environmental chamber containing substrate 50. It iscritical that the protective thermal layer be smooth to controllablyreflect radiative energy away from substrate 50 and into the gas stream.Otherwise, the radiative energy may be scattered or reflected towardunintended regions of the engine, with adverse results. Upon contactingsubstrate 50, the tantalum ethoxide deposits a tantalum oxide layer 58and ethanol by-products. Similarly, a platinum oxide layer 60 is formedby chemical vapor deposition which is then followed by application of asecond tantalum oxide layer 58 that is applied over platinum oxide layer60. The sandwiching tantalum oxide layers 58 add stability to platinumoxide layer 60, especially at higher temperatures. The ethanolby-products, being volatile, are readily removed. Other noble metallayers that may be applied, in addition to platinum and tantalum,include palladium and rhodium. Other protective layers that may beapplied in addition to tantalum oxide are titanium oxide, silicon oxide,zirconium oxide, hafnium oxide, aluminum oxide, chromium oxide andmixtures thereof.

[0024] Successful exposure testing of coupons, typically lengths ofmaterial approximately one inch in diameter, have been conducted. Suchtesting typically consists of exposing the coupon to a heat-up periodfrom ambient to a first desired temperature level, requiring a timeinterval, such as about twenty minutes, holding the coupon at the firstdesired temperature level for another time interval, such as about fortyminutes, before cooling-down the part in a manner similar to theheating-up period, and repeating these heat-up cool-down cycles to/fromthe first temperature for a predetermined number of times, typicallyseveral hundred. If the coupon survives the first temperature, thetemperature is raised by some increment, typically several hundreddegrees, and similarly cycled until the coupon coating spalls. Thepresent invention has exceeded about 1,000° C. (1800° F.) which istypically the upper temperature range of temperature an afterburner sealwill see in service.

[0025] While the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

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 10. A superalloy afterburner seal having high temperature capability comprising: an additive layer of alumina applied to an exposed surface of the superalloy seal; and a noble metal layer applied over the alumina providing a highly reflective layer, the seal further characterized by a substantial absence of a subsequent ceramic layer.
 11. The afterburner seal of claim 10 wherein the noble metal layer is selected from a group consisting of platinum, palladium and rhodium.
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